Interferometer employing a multi-waveguide optical loop path and fiber optic rotation rate sensor employing same

ABSTRACT

An interferometer employed, in part, as a Sagnac interferometer or fiber optic gyro (FOG) includes a light source ( 100 ) that provides a source light wave that is split into first and second light waves that are directed to traverse a defined optical loop path ( 508, 500 ) in opposite directions. The defined optical loop path ( 508, 500 ) in accordance with the present invention is provided by multiple waveguides wound into a coil such that the opposite traveling first and second light waves serially travel through all of the waveguides in opposite directions around the optical loop path.

FIELD OF THE INVENTION

The present invention relates, in general, to a Sagnac Interferometer,and more particularly one that is useful in a fiber optic rotation ratesensor, commonly referred to as a fiber optic gyroscope (FOG), employinga wound coil of many turns of a continuous optical fiber, defining anoptical loop path, for passing a light wave therethrough.

BACKGROUND OF THE PRESENT INVENTION

The heart of the fiber optic rotation rate sensor or gyroscope(hereafter, simply FOG) is well known in the art is the Sagnacinterferometer, briefly illustrated in FIG. 1. The Sagnac interferometerin its simplest form consists of a single directional coupler 108, e.g.a beam splitter, and an optical path that in our case amounts to alength of optical fiber formed into a loop 20, 1 turn or multiple turns,respectively. The directional coupler 108 is used to split an impinginglight wave derived from a light source 100 into two waves, but it canalso be used to combine two waves, or in other words, interfere twowaves. The directional coupler in a Sagnac interferometer will do both.After the directional coupler 108 splits the light wave, the resultingtwo waves enter opposite ends of the optical fiber loop 20 and propagatein opposite directions through the optical fiber loop 20, pass througheach other, emerge from the fiber, and are combined by the samedirectional coupler 108. The optical power of the combined waves dependsupon the phase difference between the two interfering waves. The phaseshift is related to the rotation rate of the fiber loop according to theSagnac effect, and can be expressed as,

${\Delta\varphi} = {\frac{2\pi \; {LD}}{\lambda \; c}\Omega}$

where L is the length of the fiber in the loop, D is the diameter of theloop, λ and c are the wavelength and speed of the light in vacuum,respectively. The diameter D of the loop is usually constrained by theapplication, and is typically between 1 and 6 inches. Winding theoptical fiber (hereafter referred to as simply fiber) into a multiturncoil can increase the length of fiber, L. while maintaining a manageablediameter D. Typical lengths are between 0.05 and 5 km. The packagingvolume, of course, imposes a limit on the length of the coil of opticalfiber. The outside diameter (OD) and the height of the fiber coil aregenerally limited by the space allocated by the system designer. Thefiber length is limited by the number of turns of fiber that will fit oneach layer and the number of layers that will fit in the package. Theoutside layer will provide the most signal because it has the largestdiameter as well as the largest length of fiber per layer. Eachsuccessive inner layer becomes smaller in both diameter and fiber lengthper layer. Reducing the fiber diameter increases the length of fiberthat will fit into the space allotted, but the fiber becomes moredelicate and increasingly difficult to wind accurately as the size isreduced. Accurate winding helps to ensure tight packing of the fiber,and is intended to reduce sensitivity to environmental changes such astemperature and pressure, as is well known.

Operation of the FOG is based on the Sagnac effect as aforesaid. The waythat the FOG enhances the sensitivity to rotation is via increasing thetotal phase shift between the counter-propagating waves by making themfollow a long fiber path—typical gyro fiber lengths are 50 meters to afew kilometers. The basic FOG architecture as illustrated in FIG. 1 iscalled the “Minimum” or “reciprocal” configuration. This has been theoptical architecture of choice for all medium- and high-accuracy fiberoptic gyroscopes since the 1980s (Ulrich 1980). The reason is that theclockwise and counter-clockwise waves traverse almost identical paths sothat common-mode errors tend to cancel each other. Soon after Ulrich'sproposal, the group led by Arditty and Lefevre in France proposed thehybrid architecture, which combines fiber components with an integratedoptic Y-junction fabricated with lithium niobate waveguides (Arditty etal. 1984; Lefevre et al, 1985). In a FOG, the light from the source issplit by a splitter, and the two light waves thus created enter oppositeends of an optical fiber that is wound into a coil, they pass througheach other as they counter propagate through the fiber and return to thesplitter where the two waves are combined, and the combined waves aredirected to a detector.

A cylindrical fiber coil is often referred to in terms of its diameter,D, and the length, L, of the fiber. The phase difference between thecombined waves is proportional to the LD product times the rotation rateaccording to the Sagnac effect. The main benefit of using an opticalfiber is that L can be quite large while D can be a manageable size.Because the two waves counter-propagate through the same fiber, atremendous error reduction is gained regarding perturbations of thefiber. This reduction is not perfect because the two waves traverse agiven section of fiber at different times.

The FOG has been very successful in a limited number of applications.These include applications where the fiber coil is not subjected tolarge variations in temperature and where the FOG can be made quitelarge. In some applications with limited temperature range, where thepower requirements and startup requirements allow, the fiber-optic coilis actually temperature controlled. In such cases the fiber can be madevery long and the rotation-rate signal made large compared to noise toimprove the rotation-rate measurement. The increased fiber length onlyworks in benign conditions or when the temperature is controlled becauseerrors due to environmental perturbations scale with length. Cleverwinding techniques have, however, reduced the impact of time varyingstrain gradients in the coil, but further improvement is necessary forthe FOG to reach its full potential.

If the FOG is designed and constructed well, its ability to accuratelymeasure rotation rate is limited by the stability of the presentstate-of-the-art fiber coil as it is subjected to a changingenvironment. Generally, the fiber coil is a composite structure of glassand a variety of plastics including adhesive to hold it together. Thesignal in the FOG increases with the diameter of the coil and also withthe number of turns of fiber in the coil. Increasing the diameter of thecoil has the obvious problem of increasing the size of the FOG. Thediameter of the coil is made to be as large as is acceptable in theapplication for which the FOG is intended. Increasing the number ofturns of fiber also increases the size of the coil, and it reduces thecoil stability. Adding more turns either means increasing the axialdimension of the coil or adding more layers of fiber. The plastic in thecoil expands with temperature much faster than the glass fiber. Theresult is that the plastic strains the fiber in a way that varies fromlayer to layer. This causes a time-varying strain gradient if thetemperature is varying in time, and thereby causes a measurement error.In some cases the error can be measured and compensated for to someextent; in other cases it is not easy to distinguish the error fromrotation-rate measurement, and it is not possible to compensate for theerror.

FOG Basics

FOGs have been extensively developed over the past 30 years and havedemonstrated tremendous performance. The basic design of the fibergyroscope is well documented elsewhere [Shaw; Lefreve; Bergh] and wewill not give a complete tutorial here except to mention that it is veryimportant to make use of the 30 years of development to reduce errors.In particular the design must adequately address many topics including:amongst others:

a) Reciprocity, that requires the use of a single mode filter at theinput/output port of the interferometer and proper optical polarizationmanagement;b) Rayleigh backscattering from the glass in the optical fiber; and backreflections from other sources such as fiber ends and coupling points;c) Optical Kerr effect due to the nonlinear nature of the optical fiber;d) Coherence of light throughout the gyroscope;e) Time varying gradients of temperature and strain within the fibercoil and various fiber pigtails;f) Signal processing electronics including a dual closed-loop techniqueto derive feedback modulation for the purposes of accuratelycompensating for the Sagnac phase difference and for performing areal-time calibration of the electronics and the phase modulator.

A schematic of an example of a high quality FOG is shown in FIG. 1, asearlier decibed. The interferometer portion consists of a multiturnfiber coil 150 that forms, in part, an optical loop 20, a phasemodulator 112, and a directional coupler 108. Light enters and exits theinterferometer through a single-mode single-polarization filter 106 andfiber 126. The light is delivered from a source 100, and to a detector102, through a second directional coupler, dc2, 104. In some cases acirculator is used in place of the dc2 to increase optical power at thedetector. Signal processing electronics 114 receives the detected signaland derives a feedback signal to control the phase difference betweenthe two waves exiting the interferometer. The signal processingelectronics 114 also determines the rotation rate of the gyroscope andproduces the measurement output,116.

The light source is typically an amplified stimulated emission (ASE)source. Often a polarizer 106, directional coupler 108, and phasemodulator 112 are combined in one device called a multifunction chip. Afiber coil generally provides the optical path and typically consists ofabout 1000 to 10,000 turns of fiber. The phase difference due torotation is thus increased 1000 to 10,000-fold, respectively.

As is well understood, improved performance in FOGs is gained byincreasing the total length of fiber used in the gyro and the diameterof the fiber-optic coil. The quest for ever larger signal can result invery large diameter FOGs that become challenging to package and tostabilize in a dynamic environment.

It is therefore an object of the present invention to therefore providea FOG with an improved fiber coil with improved performance with noincrease in size.

Another object of the present invention is to provide an improved Sagnacinterferometer with improved performance with no increase in size.

BRIEF SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a Sagnacinterferometer employed, in part, as a fiber optic gyro or FOG includesa light source that provides a source light wave that is split intofirst and second light waves that are directed to traverse a definedoptical loop path in opposite directions. The defined optical loop pathin accordance with the present invention is provided by a fiber coilconsisting of a fiber having many turns of an optical fiber having 2 ormore waveguides or optical cores, each having first and secondterminating ends. Further, the fiber coil is optically configured suchthat (i) a first light wave travels in a first direction through thefiber coil along an optical loop path via entering at a firstterminating end of a first waveguide, through each of the remainingwaveguides, and exiting from the second end of a designated lastwaveguide, and (ii) a second light wave travels through the fiber coilin an opposite direction and along the same optical loop path as saidfirst light wave, via entering said fiber coil at the second end of thedesignated last waveguide, through each of the remaining waveguides, andexiting from said first end of said first waveguide. In turn, theexiting light waves from the fiber coil, having traversed the sameoptical path in opposite directions, are directed to interfere in orderto determine any rotation rate induced Sagnac phase shift as is wellknown and understood.

In accordance with another embodiment of the present invention, theaforesaid optical loop path of the FOG or Sagnac interferometer isconstructed from a grouping of a plurality of waveguides into a bundlethat is wound into a coil. The bundle is then optically configured suchthat (i) a first light wave entering at a first end of a first waveguideof the bundle will travel in one direction through the fiber coil,through each of the remaining waveguides, and exit from an exit end, ofa designated exit, waveguide, and ii) a second light wave through thebundle of waveguides in an opposite direction and along the same opticalloop path as said first light wave, via entering at the exit end of thedesignated exit waveguide of the bundle, through the remainingwaveguides of the waveguide bundle, and exiting from the first end ofthe first waveguide. In turn, the exiting light waves from the waveguidebundle are directed to interfere in order to determine any rotation rateinduced Sagnac phase shift as is also well known and understood. Sinceeach waveguide of the bundle has first and opposite second ends, thewaveguides are optically coupled to each other such that light will betransferred between the second end of the first waveguide and the firstend of a second waveguide; and between the second end of the secondwaveguide and the first end of a third waveguide; and so on until lightis transferred between the second end of a next to last waveguide andthe first end of the last remaining waveguide. Light will be transferredinto and out of the coil of the waveguide bundle through, or near, thefirst end of the first waveguide and the exit end of the designated exitwaveguide.

In accordance with the embodiments as just described, the innovative FOGarchitecture employing the novel optical fiber coil construction ofmultiple waveguides will increase the Sagnac phase difference in a FOGwhile reducing the physical length of the multi-waveguide fiber in thefiber coil. Reducing the fiber length of the fiber coil results in asmaller size coil and improved thermal performance characteristics.Further, using multiple fiber cores or waveguides or bundle ofwaveguides increases the Sagnac phase difference for a given length offiber by a factor equal to the number of cores or waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fiber optic gyroscope (FOG)well known in the art employing the principles of the basic Sagnacinterferometer.

FIG. 2 is a scaled version of a cross-section of both a standardsingle-mode fiber (left) and a cross-section of a multicore fiber(right).

FIG. 3 is a scaled version of a cross-section of a multicore fiberhaving 10 concentric cores and a center core.

FIG. 4 is another scaled version of a cross-section of a multicore fiberhaving 4 cores symmetrically arranged at the corners of a squareequidistant from the center of the fiber.

FIG. 5 is a schematic block diagram of a FOG employing a multicore fibercoil in accordance with the present invention.

FIG. 6 is a schematic block diagram of a FOG employing a multicore fibercoil in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A general block diagram of a FOG known in the prior art is depicted inFIG. 1 as aforedescribed in the Background. As is common in fiber opticrate sensors, a light source 100 provides a source light wave that ispassed through a directional coupler 104, polarizer 106 via a singlemode fiber 126, and a second directional coupler 108 that splits theimpinging source light wave into (i) a first light wave that enters afirst end 129 of a single mode fiber that is wound into a fiber coil 150via fiber 128, and (ii) a second light wave that passes through a phasemodulator 112 and fiber 130 before entering the second end 131 of thesingle mode fiber that is wound into the fiber coil 150.

The first and second light waves, having traversed the same exactoptical path of the single mode fiber or single waveguide of the fibercoil 150, are again passed through the directional couple 108, polarizer106, and directional coupler 104, where a portion of each of the lightwaves is directed to impinge upon a light detector where they interfere.In turn, the detector provides an output signal for signal processing114 from which signals are derived for providing an indication ofrotation rate, as well as for proving a signal utilized for phasemodulating the first and second waves to assist in the overall signalprocessing as is well known. It should be understood, herein, that thereare a wide variety of signal processing techniques that may be employedin practice of the present invention, all of which are intended to bewithin the true spirit and scope of the present invention and will notbe described further herein.

In FIG. 2, there shown is a cross-section of a single mode optical fiber200 (on the left) that is commonly employed in providing fiber coil 150.As illustrated by way of example, fiber 200 is a scaled version of afiber with a 125-micrometer diameter and an 8-micrometer core. To usethis type of fiber, polarization management requires the use ofdepolarization within the fiber coil. Commonly, a broadband source suchas a fiber ASE source and Lyot depolarizers are employed. Great strideshave been made in depolarization in FOG technology. In this type offiber 200, the guided optical mode extends only a short distance outsideof the core of the fiber. The light thus occupies a small fraction ofthe glass of the fiber. This extra glass is there to make the fibereasier to handle. In addition, the typical telecommunications fiber isalso coated with an acrylate jacket that doubles its size as is wellknown.

As is also well know in the art of FOGs, the coil may be wound in mannerutilizing techniques to minimize stress and strain on the optical fiberby such as, e.g., a designated winding pattern as the coil isconstructed. These and other coil winding techniques, too, are allwithin the true spirit and scope of the present invention and will notbe described herein.

In accordance with the present invention, the FOG, more specifically theSagnac interferometer, is constructed having an optical loop path thatis defined by a bundle of waveguides that may be implemented by way of amulticore waveguide or the like. In the following exposition, first acommon single mode optical fiber will be illustrated along with amulticore fiber (FIGS. 2-4), and several examples thereof. In turn, FOGarchitecture employing a multicore fiber or wave guide bundle opticalloop path will be described with reference to FIGS. 5 and 6, and theirrelation to the prior art FOG architecture already described withreference to FIG. 1.

Again referring to FIG. 2, thereshown is a cross-section of a multicoreor multi-waveguide optical fiber 220 having 11 fiber cores providingmultiple optical waveguides. In FIG. 2, optical fiber 220 is illustratedas having 11 cores numerically designated as 221 a, 221 b, . . . and221N that are evenly distributed. Multicore Fiber 220 has manysimilarities to the standard single-core fiber 200. The outside diameteris still 125 micrometers, and each of the 11 cores has a diameter of 8micrometers. These 11 waveguides are also single mode. Again,depolarization will be required, as these waveguides are not designed tomaintain the input state-of-polarization. It should be noted that theaforesaid dimensions are only exemplary.

FIG. 3 illustrates yet another example of a multicore optical fiber 300.Thereshown, the center waveguide can be made to have a similarpropagation constant to the side waveguides to make an efficient couplerfrom a side to center waveguide for coupling in and out of the fiberoptical loop path. It can then be spliced to standard single-mode fiberto transfer light from the center waveguide in the multicore fiber tothe only waveguide in the standard single-mode fiber.

It should be noted, that the cross-sectional geometry of multicorefibers for FOG applications may be optimized using simulations todetermine practical limits on the number of cores and their optimumpositions. Available multicore fibers consisting of a two-core fiber arereadily available from fiber manufacturers. Multicore fiber may befabricated by manufacturers such as Polymicro, Corning, OFS,Stocker-Yale or Verrillon for the fabrication of fibers with four ormore cores. One example of a multicore optical fiber is disclosed inU.S. Pat. No. 6,301,420, entitled, “Multicore Optical Fibre”, issued toGreenaway, et. al.

It should be further noted that multicore fiber for sensors applicationshas been receiving increasing attention. For example, France Telecom[Bethuys et al., 1998] has provided multicore fiber for Fiber BraggGrating (FBG) based bend sensor applications [Cranch et al, (NRL) 2006;Flockhart et al., 2003]. Dr. Bob Rogowski's group at NASA LangleyResearch Center has produced multicore fiber 400, shown in FIG. 4, byinserting Ge-doped rod into holes drilled in a silica preform before thefiber was drawn [Fender et al]. Applicant had been already working withsamples of multicore fiber from NASA Langely Research Center.

As indicated earlier, a multi-waveguide fiber coil maybe constructed byway of wide variety of winding techniques that may improve FOGperformance characteristics, more or less. Different winding patterns,as aforesaid may include, among others, quadrupole, hexadecapole, andinterleave, all of which are intended to be within the scope of thepresent invention.

Gyro Architecture

Illustrated in FIG. 5 is a schematic block diagram of a FOG architecturediffering somewhat to that shown in FIG. 1 to take advantage of amuilt-wavgeguide fiber that make up a fiber coil in accordance with thepresent invention. In FIG. 5, like components having similar function asthose in FIG. 1 have retained the same numeral designation. Inparticular, a multicore fiber 508 providing multiple waveguides isutilized for the construction of the fiber coil 500. Further, aMulticore-to-standard Fiber Directional Couple 502 (msdc 502) isemployed for coupling light waves into and out of fiber coil 500.

As illustrated, the pair of light waves emanating from the splitter 108is coupled to the multicore fiber 508 through standard fibers 504 and506. The msdc 502, in turn couples light into and out of fiber coil fromfiber ends 508 a and 508 b.

As further illustrated in FIG. 5, a multicore fiber 508, similar tothose illustrated in FIGS. 2-4, provides fiber coil 500 that providesmultiple waveguides as already described. Appropriate splicing of thefiber cores of the multicore fiber 508 will be optically configured intoan optical loop path with optical core splicing that ensures that lightthat is introduced into one of the waveguides will pass through all ofthe waveguides near the perimeter of the fiber (in the case of a fiberlike that shown in FIG. 3) before returning to the spatial point ofintroduction. In the case of a prime number of waveguides near theperimeter, as in our 11-core example of FIG. 3, it is only necessary tohave a given core lined up with any core other than itself to insurethat all cores will be used. To introduce the light, side polishedcouplers may used. Preferably, the optical structure and configurationis constructed such that light does not recirculate in the optical looppath, but makes a singe pass through all the cores.

In FIG. 5, a multicore-to-standard fiber ideally will couple 100 percentof the light from a standard sing mode fiber, say fibers 504 and 506, toone of the multicore fiber waveguides, and 100 percent back again. It isquite likely in reality that the coupling will not be complete. In thiscase the design as illustrated in FIG. 6 may be more suitable.

In one embodiment of the invention as that illustrated in FIG. 5,consider by way of example, an optical loop path provided by way of anoptical fiber having 11 cores, i.e., N=11. After light propagates aroundthe fiber coil through one of the cores, it is then coupled, back at theentry point, to a second core to propagate around the fiber coil againvia multicore direction coupler 502. The light will then be coupledthrough a third core, and so on, until it has propagated through all Ncores (N times through the fiber) and emerges from the coil. The resultwill be a Sagnac phase difference between interfering waves that is Ntimes what it would be for a standard fiber with a single core. Thissignal improvement is accomplished without making the fiber longer, butby more efficient use of the space within the fiber. More signal isachieved for the same size fiber coil. Or alternatively, the signal sizeis increased even while the length of fiber in the coil, and thus thesize of the coil, is reduced. In our example, the Sagnac phasedifference is 11 times larger than it would be in a single core fiber.

Small and accurately wound fiber coils take better advantage of thenatural common mode rejection afforded by the Sagnac interferometer. Ina Sagnac interferometer, two waves are compared after they propagate inopposite directions along the same optical path. Changes in the fiberaffect both waves but at slightly different times. Accurately windingthe fiber in the coil is intended to bring part of the fiber on one sideof the fiber loop into close proximity to a part of the fiber on theother side of the fiber loop that is equidistant from the loop coupler.Any perturbation that occurs on one side will likely occur on the other,and both of the counter-propagating waves will then experience the sameperturbation at the same time and no spurious phase difference willoccur when the waves interfere. This common mode rejection works to someextent. Unfortunately the perturbation of one fiber is not exactly thesame as the neighboring fiber regardless of how accurately the fiber iswound. Additionally, it would help if the time difference between thepassing of the two counter-propagating waves could be reduced.

In a multicore fiber coil the error-inducing time difference is reducedby an order of magnitude. The two counter-propagating waves pass throughthe same section of fiber with a time difference that is at most equalto one transit time around the fiber. This is shorter than the time fora single core fiber because the fiber in the coil is shorter for a givenSagnac phase shift by a factor of N, where N is the number of coreswithin the fiber.

In FIG. 6, thereshown is a schematic block diagram of a FOG architecturediffering somewhat to that shown in FIG. 5 to take advantage of amulticore fiber or muilt-wavgeguide bundle that make up a fiber coil inaccordance with the present invention. In FIG. 6, like components havingsimilar function as those in FIG. 5 have retained the same numeraldesignation.

In FIG. 6, msdc 502 of FIG. 5 is replaced with a pair ofMulticore-to-standard Fiber Directional Couplers, msdc couplers 600 and602. Msdc coupler 600 and msdc couple 602 are also ideally 100 percent,but the possibility of light returning to the polarizer 106 withoutmaking at least one pass through the 11 cores, in the example, of themulticore fiber coil is significantly reduced.

As is well understood by those skilled in the art of the SagnacInterferometers, and FOGs in particular, it is of paramount importancethat the counter-propagating light waves that traverse the optical looppath follow exactly the same optical loop path, but in oppositedirections.

Accordingly, as indicated earlier, in accordance with the presentinvention, a Sagnac interferometer employed, in part, as a FOG includesa light source that provides a light wave that is split into first andsecond light waves that are directed to traverse a defined optical looppath in opposite directions. The defined optical loop path in accordancewith the present invention is provided by a fiber coil consisting of afiber having many turns of an optical fiber having two (2) or morewaveguides or optical cores. Further, the fiber coil is opticallyconfigured such that (i) a first light wave travels in a first directionthrough the fiber coil along an optical loop path via entering at afirst end of a first waveguide, through each of the remainingwaveguides, and exiting from an exit end of a designated exit waveguide,and (ii) a second light wave travels through the fiber coil in anopposite direction and along the same optical loop path as said firstlight wave, via entering said fiber coil at said designated exitwaveguide exit end, through each of the remaining waveguides, andexiting from said first end of said first waveguide. In turn, theexiting light waves from the fiber coil, having traversed the sameoptical path in opposite directions, are directed to interfere in orderto determine any rotation rate induced Sagnac phase shift as is wellknown and understood.

In accordance with another embodiment of the present invention, theaforesaid optical loop path of the FOG or Sagnac interferometer isconstructed from a grouping of a plurality of waveguides into a bundlethat is wound into a coil. The bundle is then optically configured suchthat (i) a first light wave entering at a first end of a first waveguideof the bundle will travel in one direction through the fiber coil,through each of the remaining waveguides, and exit from an exit end of adesignated exit waveguide, and (ii) a second light wave travels throughthe bundle of waveguides in an opposite direction and along the sameoptical loop path as said first light wave, via entering at the exit endof the designated exit waveguide of the bundle, through remainingwaveguides of the waveguide bundle, and exiting from the first end ofthe first waveguide. In turn, the exiting light waves from the waveguidebundle are directed to interfere in order to determine any rotation rateinduced Sagnac phase shift as is also well known and understood. Sinceeach waveguide of the bundle has a first and second end, the waveguidesare optically coupled to each other such that light will be transferredbetween the second end of the first waveguide and the first end of asecond waveguide; and between the second end of the second waveguide andthe first end of a third waveguide; and so on until light is transferredbetween the second end of a next to last waveguide and the first end ofthe exit or last waveguide. Light will be transferred into and out ofthe coil of the waveguide bundle through, or near, the first end of thefirst waveguide and the exit end of the designated exit waveguide.

Thus, the multiple waveguide optical loop path of the interferometermaybe provided by wide array of multicore fiber configurations asexemplified in FIGS. 2-4, and alternatively by way of a bundle ofoptical waveguides that may be facilitated and manufactured to form afiber coil using proper optical waveguide handling techniques, all ofwhich are intended to be within the true spirit and scope of the presentinvention.

Now consider the exemplary light path in the multi-waveguide fiber orbundle where a fiber has 11 (N=11) waveguides or cores. After lightpropagates around the fiber coil through one of the cores, it is thencoupled, back at the entry point, to a second core to propagate aroundthe fiber coil again. The light will then be coupled through a thirdcore, and so on, until it has propagated through all N cores (N timesthrough the fiber) and emerges from the coil. The result will be aSagnac phase difference between interfering waves that is N times whatit would be for a standard fiber with a single core. This signalimprovement is accomplished without making the fiber longer, but by moreefficient use of the space within the fiber. More signal is achieved forthe same size fiber coil. Or alternatively the signal size is increasedeven while the length of fiber in the coil, and thus the size of thecoil, is reduced. In our example, the Sagnac phase difference is 11times larger than it would be in a single core fiber.

Small and accurately wound fiber coils take better advantage of thenatural common mode rejection afforded by the Sagnac interferometer. Ina Sagnac interferometer, two waves are compared after they propagate inopposite directions along the same optical path. Changes in the fiberaffect both waves but at slightly different times. Accurately windingthe fiber in the coil is intended to bring part of the fiber on one sideof the fiber loop into close proximity to a part of the fiber on theother side of the fiber loop that is equidistant from the loop coupler.Any perturbation that occurs on one side will likely occur on the other,and both of the counter-propagating waves will then experience the sameperturbation at the same and no spurious phase difference will occurwhen the waves interfere. This common mode rejection works to someextent. Unfortunately the perturbation of one fiber is not exactly thesame as the neighboring fiber regardless of how accurately the fiber iswound. Additionally, it would help if the time difference between thepassing of the two opposite traveling light waves could be reduced.

In a multicore fiber coil the error-inducing time difference is reducedby an order of magnitude. The two counter-propagating waves pass throughthe same section of fiber with a time difference that is at most equalto one transit time around the fiber. This is shorter than the time fora single core fiber because the fiber in the coil is shorter for a givenSagnac phase shift by a factor of N, where N is the number of coreswithin the fiber.

It shall be noted that there is a wide array of optical splicingtechniques, well known in the art, to facilitated coupling of theprimary split waves that counter-propagate through the optical looppath. The FOG architecture employing one or more multicore-to-standardfiber directional couplers maybe utilized as desired depending upon, ofcourse, desired performance outcomes. Each of these alternativeembodiments are, of course, all intended to be within the true spiritand scope of the present invention, and well known to the artisan.

Lastly, the optical loop path of Sagnac Interferometer using a multimodeor multicore or multi-waveguide architecture may be employed in a widearray of sensor applications, including among others, current andvoltage sensors, all of which are intended to be within the true spiritand scope of the present invention.

1. A fiber optic interferometer comprising: a light source (100) thatprovides a source light wave; a light wave splitter (108) that splitssaid source light wave into first and second light waves; and opticalmeans (602, 600,602) for introducing said first and second light wavesto travel along an optical loop path and exit therefrom, where, saidoptical loop path (508, 500) is constructed by two or more opticalwaveguides that form a loop of one or more turns about a reference axispassing therethrough, wherein said two or more optical waveguides eachhave respective first and second terminating ends, and said first andsecond terminating ends of said waveguides are selectively opticallycoupled to each other such that said first and first light waves maytravel in opposite directions along an identical optical loop paththrough each of said two or more waveguides before exiting said opticalloop path.
 2. The interferometer of claim 1 serves, in part, as a Sagnacinterferometer rotation rate sensor, and further includes a lightdetector (102) for responding to a portion of said first and secondlight waves exiting said optical loop path.
 3. The interferometer ofclaim 1 serves, in part, as a fiber optic rotation rate sensor fordetecting rotation of said optical loop path about said reference axis,the interferometer further including a light detector (102) responsiveto a portion of said first and second light waves exiting said opticalloop path for providing an output signal indicative thereof related tothe phase difference between the first and second light waves travelingalong said optical closed loop path induced by rotation of said opticalloop path.
 4. A fiber optic interferometer comprising; a light source(100) that provides a source light wave; a light wave splitter (502)that splits said source light wave into first and second light waves;and optical means for introducing said first and second light waves totravel along an optical loop path (500, 508) and exit therefrom, whereinsaid optical loop path (500,508) is provided by a wound optical fibercoil that forms a loop of one or more turns about a reference axispassing therethrough, and said optical fiber coil (500) is constructedfrom a multicore optical fiber establishing two or more opticalwaveguide cores, where each waveguide core has respective first andsecond terminating ends selectively optically coupled to each other suchthat said first and second light waves may travel in opposite directionsalong an identical optical loop path through each of said two or morewaveguide cores before exiting said optical loop path.
 5. Theinterferometer of claim 4 serves, in part, as a Sagnac interferometerrotation rate sensor, and further includes a light detector (102) forresponding to a portion of said first and second light waves exitingsaid optical loop path.
 6. The interferometer of claim 4 serves, inpart, as a fiber optic rotation rate sensor for detecting rotation ofsaid optical loop path about said reference axis, the interferometerfurther including a light detector (102) responsive to a portion of saidfirst and second light waves exiting said optical loop path forproviding an output signal indicative thereof related to the phasedifference between the first and second light waves traveling saidoptical closed loop path induced by rotation of said optical loop path.7. A fiber optic interferometer comprising: a light source (100) thatprovides a source light wave; a light wave splitter (108) that splitssaid source light wave into first and second light waves; and opticalmeans (502, 600, 602) for introducing said first and second light wavesto travel along an optical loop path (500, 508) and exit therefrom,wherein said optical loop path is provided by a wound optical fiber coilthat forms a loop of one or more turns about a reference axis passingtherethrough, and said optical fiber coil (500) is a wound bundle of twoor more optical waveguides that forms a loop of one or more turns abouta reference axis passing therethrough, where each waveguide of saidbundle has respective first and second terminating ends selectivelyoptically coupled to each other such that said first and second lightwaves may travel in opposite directions along an identical optical looppath through each of said two or more waveguides before exiting saidoptical loop path.
 8. The interferometer of claim 7 serves, in part, asa Sagnac interferometer rotation rate sensor, and further includes alight detector (102) for responding to a portion of said first andsecond light waves exiting said optical loop path.
 9. The interferometerof claim 7 serving, in part; as a fiber optic rotation rate sensor fordetecting rotation of said optical loop path about said reference axis,the interferometer further including a light detector (102) responsiveto a portion of said first and second light waves exiting said opticalloop path for providing an output signal indicative thereof related tothe phase difference between the first and second light waves travelingalong said optical closed loop path induced by rotation of said opticalloop path.
 10. An fiber optic rotation rate sensor comprising: a lightsource (100) that provides a first light wave; a light wave splitter(108) that splits said first light wave into a first and a second lightwave; and optical means for introducing said first and second lightwaves to travel along an optical loop path (500, 508)) and exittherefrom, wherein said optical loop path is provided by a grouping of aplurality of waveguides, each having first and second ends, into abundle that is wound into a coil, where the bundle is then opticallyconfigured such that, (i) light waves will be transferred between thesecond end of the first waveguide and the first end of a secondwaveguide; and (ii) between the second end of the second waveguide andthe first end of a third waveguide; and so on until light is transferredbetween the second end of a next to last waveguide, and the first end ofthe last or exit waveguide, and light will be transferred into and outof the coil of the waveguide bundle through, or near, the first end ofthe first waveguide and the second end of the designated last or exitwaveguide, where, in turn, the exiting light waves from the waveguidebundle are directed to interfere in order to determine any rotation rateinduced Sagnac phase shift; optical means (502, 106,108,104) fordirecting at least a portion of said first and second light wavesexiting said optical loop path to impinge upon a detector (102) andproducing a detector output signal indicative of interfering first andsecond light waves impinging thereon; and signal processing means (114)for providing an output signal (116) indicative the of rotation rate ofsaid optical loop path about said reference axis.
 11. The fiber opticrotation rate sensor of claim 10 further comprising: optical means (112)for modulating the phase of said first and second light waves travelingalong said optical loop path in response to a modulation signal; andwherein said signal processing means (114) provides said modulationsignal and demodulates said detector output signal as a function of saidmodulation signal so as to derive said signal indicative of the rotationrate.
 12. A fiber optic rotation rate sensor comprising: a light source(100) that provides a source light wave; a light wave splitter (108)that splits said source light wave into first and second light waves;optical means (502,600,602) for introducing said first and second lightwaves to travel along an optical loop path and exit therefrom, where,said optical loop path is constructed by two or more optical waveguidesthat form a loop of one or more turns about a reference axis passingtherethrough, wherein said two or more optical waveguides each haverespective first and second terminating ends, and said first and secondterminating ends of said waveguides are selectively optically coupled toeach other such that said first and first light waves may travel inopposite directions along an identical optical loop path through each ofsaid two or more waveguides before exiting said optical loop path;optical means (502, 106,108,104) for directing at least a portion ofsaid first and second light waves exiting said optical loop path toimpinge upon a detector (102) and producing a detector output signalindicative, of interfering first and second light waves impingingthereon; and signal processing means (114) for providing an outputsignal (116) indicative the of rotation rate of said optical loop pathabout said reference axis.
 13. The fiber optic rotation rate sensor ofclaim 12 further comprising, optical means (112) for modulating thephase of said first and second light waves traveling along said opticalloop path in response to a modulation signal; and wherein said signalprocessing means (114) provides said modulation signal and demodulatessaid detector output signal (102) as a function of said modulationsignal so as to derive said signal indicative of the rotation rate. 14.A fiber optic rotation rate sensor comprising: a light source (100) thatprovides a source light wave; a light wave splitter (108) that splitssaid source light wave into first and second light waves; optical means(502, 600,602) for introducing said first and second light waves totravel along an optical loop path and exit therefrom, wherein saidoptical loop path is provided by a wound optical fiber coil that forms aloop of one or more turns about a reference axis passing therethrough,and said optical fiber coil is constructed from a multicore opticalfiber establishing two or more optical waveguide cores, where eachwaveguide core has respective first and second terminating endsselectively optically coupled to each other such that said first andsecond light waves may travel in opposite directions along an identicaloptical loop path through each of said two or more waveguide coresbefore exiting said optical loop path; optical means (502, 108, 106,104) for directing at least a portion of said first and second lightwaves exiting said optical loop path to impinge upon a detector (102)and producing a detector output signal indicative of interfering firstand second light waves impinging thereon; and signal processing means(114) for providing an output signal indicative the of rotation rate ofsaid optical loop path about said reference axis.
 15. The fiber opticrotation rate sensor of claim 14 further comprising: optical means (112)for modulating the phase of said first and second light waves travelingalong said optical loop path in response to a modulation signal; andwherein said signal processing means (114) provides said modulationsignal and demodulates said detector output signal as a function of saidmodulation signal so as to derive said signal indicative of the rotationrate.